"I told them I was a white man who hated n--, spics, Chinks, Jews, Japs and anybody who wasn't pure Aryan white like I was," Stallworth recalled during a recent phone interview. "I told them I wanted to do something to stop the abuse of the white race."

But Stallworth made a mistake. "I signed my real name, instead of my undercover name," he said, "and mailed it off, thinking I would get a pamphlet, brochure or something."

Two weeks later, the phone rang at Stallworth's desk.

It was the organizer for the local Klan chapter in Colorado Springs.

He asked, Stallworth said, "why I wanted to join 'The Organization,' as they called it."

Stallworth, who was born in Chicago and grew up in El Paso, thought about all the times in his life he'd been called the n-word. He thought about the most vile, hate-filled response he could give to impress a Klan member.

Then he reeled off the same string of ugly pejoratives for blacks, Jews, Asians, Hispanics and "anyone else that does not have pure white Aryan blood in their veins."

"I told him my sister had been dating a n-- and every time he put his filthy black hands on her pure white body it made me cringe, and I wanted to do something to stop those things from happening."

The Klan organizer was delighted: "You are just the kind of guy we are looking for. When can we meet?"

"I had to formulate a plan real quick," Stallworth said. "I told him I couldn't meet him now. We agreed to meet a week later. I started putting things in motion, getting a white officer to pose as me for this face-to-face meeting."

With that, Stallworth began a seven-month undercover investigation, infiltrating the Klan with the help of the white colleague who pretended to be him. He duped KKK Grand Wizard David Duke, whom Stallworth called regularly to get information on Klan activity. Duke had no idea the man he was talking to on the other end was black.

The story of Stallworth's investigation is featured in the movie "BlacKkKlansman," which is based on Stallworth's book "Black Klansman: Race, Hate, and the Undercover Investigation of a Lifetime."

The movie opens in theaters nationwide Friday to mark the first anniversary of the violent Unite the Right rally in Charlottesville. A 32-year-old counterprotester, Heather D. Heyer, was killed after an avowed white supremacist allegedly struck a crowd with his car.

The movie, directed by Spike Lee and produced by Jordan Peele, stars Adam Driver as the white Ron Stallworth and John David Washington, the son of Denzel Washington, playing black Ron Stallworth. It won the Grand Prix at the Cannes Film Festival.

"I've seen the movie twice," said Stallworth, now 65 and living in El Paso. "It is a very powerful movie. Obviously, Spike wove the story around my story. He did a good job putting the story together and connecting the historical thread from the Confederacy to Charlottesville, David Duke and Donald Trump."

The movie, Stallworth said, is timely and symbolic: "People need to realize this is a threat to the very fabric of American society."

That threat was very real in Colorado in 1978, when Stallworth began his investigation, which would prevent three cross burnings, reveal Klan members in the military and the North American Aerospace Defense Command, and thwart a plan by Klan members to bomb two gay bars in Denver.

During the investigation, Stallworth reviewed secret FBI files on the history of the Klan in Colorado, which organized there in about 1921. Just two years later, it was estimated the Klan in Colorado had 35,000 to 40,000 members.

The files contained information on Klan bombings, including the home of a black postal worker who moved into a white neighborhood. In 1925, the Klan allegedly burned down Shorter Community African Methodist Episcopal Church in Denver.

The Klan dominated the Colorado State Senate and House of Representatives and counted longtime Denver Mayor Benjamin Stapleton among its members.

"Not only was Stapleton a member of the Ku Klux Klan in 1923 when he was first elected mayor, but once in office he bowed to pressure from the Klan to appoint its pick for police chief," according to the Denver Post. "When Stapleton survived a recall effort, the Klan burned a cross on the top of Table Mountain to celebrate."

A movement has begun to remove Stapleton's name from buildings and the community, according to the Colorado Independent newspaper. The city's first international airport was also named for Stapleton before its closure in 1995, and Stallworth wonders how many people who flew in and out of that hub knew "that, in their own way, they [were] paying homage to a past leader of the Ku Klux Klan."

Colorado Gov. Clarence J. Morley was a Klansman, according to the Denver Public Library. Two U.S. senators, Rice Means and Lawrence Phipps, had strong Klan connections, as did the lieutenant governor, state auditor and attorney general. William J. Candlish, a grand dragon, was chief of the Denver Police Department. Klan members sat on the Board of Regents for the University of Colorado and the State Supreme Court, Stallworth discovered.

This information fueled Stallworth's determination to stop any "domestic terrorism" perpetuated by the Klan in Colorado.

One day, as part of the investigation, Stallworth came across a phone number for "The Voice of the Klan."

Stallworth dialed the number. He wasn't expecting what happened next.

"Damned if David didn't pick up the line," Stallworth recalled, calling Duke by his first name. "He laughed and said, 'I'm 'The Voice of the Klan.' He identified himself as the grand wizard, the director."

"I told him I was a new Colorado Springs chapter member and I was honored to speak with him," Stallworth said.

In follow-up calls, Stallworth showered Duke with compliments. "He was very much like Donald Trump in the sense he liked to be fawned over," Stallworth said. "He liked flattery. I played that game and flattered him a lot."

Duke was pleasant, Stallworth said, "when he wasn't talking race. But inevitably the subject of race would come up. Dr. Jekyll turned into Mr. Hyde and the monster in him came out, and he said all the vile things about racial groups and people of color, ethnicities and people, as they put it, who were not pure Aryan white."

Duke never figured out he was talking to a black detective.

"It was hard to keep a straight face and not bust out laughing," Stallworth said.

During one conversation, Stallworth recalled, "I said, 'Mr. Duke, aren't you ever worried that some smart aleck n-- would ring you up and pretend to be white in order to find some information about the Klan?' "

"He said, 'No, I never worry about that.' He said, 'I can always tell when I'm talking to a n-- on the phone, and I would be able to be able to tell they weren't pure Aryan white.' "

Stallworth asked how.

Duke explained that he could decipher color over the phone line, " 'by the way they pronounce certain words and phrases.' "

"I said, 'What do you mean?' " Stallworth recalled. "He said, 'Take, for example, you, I can tell you are pure Aryan white because you pronounce the English language the way it was meant to be pronounced. A n-- could not do that.' "

Duke has denied Stallworth's account, tweeting: "This whole KKK film is a big lie. Cops infiltrated a Klan chapter. But, if the film is true by depicting them as violent, why no arrests? no trials? no grand juries? Why? They were NOT violent or illegal - Only trying to awaken whites to their coming ethnic cleansing in America."

Stallworth said people have often asked him why no arrests were made. "That was a always a bone of contention," he said. "Around law enforcement circles and among people like David Duke who try to downplay it. Nobody was arrested for a criminal offense."

But Stallworth said he was conducting an "intelligence investigation. From intelligence standpoint, we were successful. No cross burnings in 7½ months during this investigation. There were not because I was invited to participate in the cross burning." When Stallworth got a tip, the police department would send police cars to the location of the planned cross burning.

"They talked about bombing the two gay bars," Stallworth said. "But they didn't. We stopped two Klansmen whose job in the military was to deal with explosives from carrying out a threat to bomb. They talked about stealing automatic weapons from Fort Carson Army base to stock in preparation for a race war. We gained valuable intelligence."

When Duke made a trip to Colorado Springs for a Klan rally, Stallworth was given an assignment he could not believe: His supervisor assigned him to guard Duke.

During the visit, Duke never recognized the man he had been talking to for so many months was black. At the Bonanza Steakhouse in Colorado Springs, Stallworth asked Duke whether they could have a photo together. Duke agreed, until Stallworth placed his hand on Duke's shoulders and asked the "white Ron Stallworth" to snap a Polaroid.

"Duke ran to get it out of my hand," Stallworth recalled. "I told him, 'If you touch me, I will arrest you for assault of a police officer. That is worth about five years in prison. Don't do it.' He stood there dumbfounded."

In March 1979, after the local organizer of the Colorado Springs Klan insisted that "Ron Stallworth" take over as leader, Stallworth was ordered by the police chief to shut down the investigation. He ordered Stallworth to cease all phone calls and contact. "The chief made it clear that he wanted 'Ron Stallworth - Klansman' to completely disappear."

The chief ordered Stallworth to destroy all evidence of the investigation. Stallworth took some of the files home, including his Klan membership card with his name on it.

The polaroid of Stallworth and Duke, he said, was lost in a move.

"I haven't seen the picture in over 40 years," he said. "If I had known I would write a book, I would have taken good care of it."

After Stallworth's Klan operation ended, he spent time as a narcotics investigator and on special assignment with the Colorado attorney general's Organized Crime Strike Force.

Eventually, he worked in law enforcement in Wyoming and then for the Utah Department of Public Safety's narcotics enforcement bureau. He retired in 2005.